1. Field of the Invention
The present invention relates to a nitride semiconductor light emitting device, and more particularly, to a flip chip type nitride semiconductor light emitting device having a highly reflective ohmic contact structure which can reduce unfavorable effect of reflective metal used while enhancing reflectivity.
2. Description of the Related Art
In general, a nitride semiconductor constituting a nitride semiconductor light emitting device, more particularly, a p-type nitride semiconductor layer has a wide band gap, having a difficulty in forming an ohmic contact with an electrode. Also, an ohmic contact structure adopted in a flip chip type nitride semiconductor light emitting device is required to have high reflection property for enhancement of luminance in addition to its basic ohmic contact function.
In general, metals such as Ag and Al are mainly used for highly reflective metal. However, migration tendency of Ag has an effect on GaN dislocation, presenting a problem of possible current leakage. Al is stable but has difficulty in forming an ohmic contact. Therefore, an additional layer is needed to form an ohmic contact with the p-type nitride semiconductor.
U.S. Pat. No. 5,563,422 (patented on Oct. 8, 1996 and assigned to Nichia Chemistry Co.) proposes a transparent electrode structure which comprises a thin Ni/Au film deposited on a p-type nitride semiconductor layer, which is thermally treated so that an ohmic contact is possible. However, the transmission ratio of the transparent electrode is low at 60% to 70%, undermining light emission efficiency overall.
On the other hand, Japanese Laid-Open Publication Application No. 2000-183400 (published on Jun. 30, 2000 by Toyoda Gosei Co. LTD.) proposes a method of adopting a thin film made of Ni or Co and a reflective metal layer composed of Ag, Rh, and Pt. The application discloses a multi-level structure in which the thin film is used to form an ohmic contact, and a metal layer having high reflectivity is additionally deposited to allow high reflectivity. But, Ni or Co is metal, and does not have high transmission ratio even if it is made into a thin film, which does not allow a great level of improvement in luminance.
Therefore, a new reflective ohmic contact structure has been required in the art in order to stably prevent unfavorable effects of the reflective metal layer while maximizing the reflective effect from the reflective metal layer to improve luminance in a flip chip type nitride semiconductor light emitting device.